1. Field of the Invention
The present invention relates to a method of and an apparatus for reforming a fuel and a fuel cell system with the fuel-reforming apparatus incorporated therein. More specifically, the present invention pertains to a fuel-reforming apparatus that reforms a hydrocarbon supplied as a raw fuel to a hydrogen-rich gaseous fuel, which is then supplied to fuel cells. The present invention further pertains to a method of reforming a fuel and a fuel cell system with such a fuel-reforming apparatus incorporated therein.
2. Description of the Prior Art
Fuel cells are a device in which the chemical energy of a fuel is converted, not via mechanical energy or thermal energy, but directly into electrical energy. The fuel cells can thus realize a favorably high energy efficiency. A well-known structure of the fuel cell includes a pair of electrodes arranged across an electrolyte layer. While a gaseous fuel containing hydrogen is supplied to one electrode (cathode), an oxidizing gas containing oxygen is fed to the other electrode (anode). An electromotive force is obtained by taking advantage of electrochemical reactions proceeding at the respective electrodes. Equations representing the electrochemical reactions occurring in the fuel cell are given below. Equations (1) and (2) respectively represent the reaction at the anode and the reaction at the cathode; the reaction expressed as Equation (3) accordingly proceeds as a whole in the fuel cell: EQU H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- (1) EQU (1/2)O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O (2) EQU H.sub.2 +(1/2)O.sub.2 .fwdarw.H.sub.2 O (3)
Fuel cells are generally classified by the type of the electrolyte used therein, the operation temperature, and the other parameters. Among the various fuel cells, Polymer Electrolyte Fuel Cells, Phosphoric Acid Fuel Cells, and Molten Carbonate Fuel Cells allow supplies of the oxidizing gas and the gaseous fuel containing carbon dioxide, because of the characteristics of their electrolytes. In these fuel cells, the air is generally used as the oxidizing gas, and the hydrogen-containing gas obtained by steam reforming a raw hydrocarbon fuel, such as methanol or natural gas as the gaseous fuel.
The fuel cell system having such fuel cells is accordingly provided with a reformer, which reforms the raw fuel to generate a gaseous fuel. The following gives an exemplified reforming reaction of the raw fuel proceeding in the reformer. In this example, methanol is supplied as the raw fuel and steam reformed: EQU CH.sub.3 OH.fwdarw.CO+2H.sub.2 -90.0 (kJ/mol) (4) EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 +40.5 (kJ/mol) (5) EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +3H.sub.2 -49.5 (kJ/mol) (6)
In the process of steam reforming methanol, the decomposition of methanol expressed as Equation (4) and the converting reaction of carbon monoxide expressed as Equation (5) occur simultaneously; the reaction expressed as Equation (6) accordingly proceeds as a whole in the reformer. Since the process of steam reforming the raw fuel is an endothermic reaction, the conventional reformer is typically provided with a burner or a heater in order to supply a required amount of heat for the reforming reaction.
In the structure including the burner to supply a required amount of heat for the reforming reaction, however, the burner itself attached to the reformer and additional conduits to feed supplies of a fuel and the air to the burner for combustion make the whole fuel cell system rather complicated and bulky. This is especially unsuitable when the fuel cell system having the fuel cells and the reformer is located in a limited space, for example, when the fuel cell system is mounted on the vehicle as a power source for driving the vehicle. The structure including the heater, on the other hand, requires extra energy for driving the heater, in addition to having the above drawbacks, that is, the complicated and bulky fuel cell system. This leads to a decrease in energy efficiency of the whole fuel cell system. By way of example, in the structure that supplies part of electric power generated by the fuel cells to the heater which is used for heating the reformer, the fuel cells are required to have a sufficiently large capacity.
The conventional reformer can not be favorably applied to the case in which an increase in supply of gaseous fuel is required with the enhanced loading of the fuel cells. The reforming reaction expressed as Equation (6) is an endothermic reaction as discussed above. The endothermic reaction generally has a slower reaction rate, and it is accordingly difficult to abruptly increase the amount of the raw fuel processed by the reforming reaction. The endothermic reforming reaction can be activated by increasing the amount of heat applied to the reformer. An extreme increase in temperature, however, deteriorates the catalyst packed in the reformer and causes other problems. Application of a small amount of heat to prevent deterioration of the catalyst leads to an insufficient effect of activating the reforming reaction. As another possible solution, a reformer of a sufficiently large capacity may be used to readily generate an estimated maximum amount of the reformed gas. This, however, makes the reformer undesirably bulky.
In the structure of heating the reformer with an external heat source, such as a heater, another problem arises; that is, the temperature distribution curve in the reformer has smaller values in the vicinity of the inlet of the reformer and greater values in the vicinity of the outlet. FIG. 21 is a graph showing a temperature distribution in a conventional reformer with a heater. In the conventional reformer, the inside temperature decreases with the progress of the endothermic reforming reaction at the inlet, through which steam and methanol as the raw fuel are introduced. Although the heater continues supplying heat, the temperature in the reformer is decreasing while the endothermic reforming reaction is vigorous to consume a large amount of heat. As the progress of the endothermic reforming reaction becomes gentle with consumption of the raw fuel, the amount of heat supplied by the heater reaches and then exceeds the amount of heat required for the endothermic reaction. The inside temperature of the reformer accordingly starts increasing. A temporary decrease in temperature in the vicinity of the inlet of the reformer lowers the rate of the endothermic reforming reaction and thereby the efficiency per unit volume of the reformer. An increase in temperature in the vicinity of the outlet interferes with the exothermic shift reaction of Equation (5), thereby undesirably increasing the concentration of carbon monoxide included in the gaseous fuel obtained by the reforming reaction.